A photographic material comprises a silver halide emulsion provided on a support. The silver halide emulsion is usually chemically sensitized with various chemical sensitizers to obtain a desired sensitivity or gradation. Examples of the chemical sensitizers include a chalcogen (sulfur, selenium or tellurium) sensitizer, a noble metal (such as gold) sensitizer, a reduction sensitizer and a combination thereof.
An improvement of the silver halide photographic material has increasingly been demanded for several years. The recent photographic material requires very high sensitivity. Further, the photographic image requires an improvement of the graininess and the sharpness. A rapid image forming process such as a quick development process is also necessary. The sensitization and the sensitizers have been improved to meet these requirements.
Sulfur sensitization has most frequently been used in the silver halide photography. Numerous sulfur sensitizers have been known and used in silver halide photographic materials.
On the other hand, selenium or tellurium sensitization is more effective than the sulfur sensitization. Accordingly, selenium or tellurium sensitizers have been proposed in place of the sulfur sensitizers. However, the proposed sensitizers have a tendency that fogs easily occur in the image and gradation of the image is softened. Therefore, the sensitizers should be further improved to be used in place of the conventional sulfur sensitizers.
U.S. Pat. No. 3,297,447 discloses selenourea derivatives (e.g., dimethylselenourea) as improved selenium sensitizers. However, the improvement is still insufficient, and a rather dense fog is observed in the image (as is shown in Tables in Examples of the present specification).
The above-mentioned chalcogen sensitization is usually used in combination with a gold sensitization. The combinations of the sensitizations remarkably improve the sensitivity of the photographic material. However, the combinations also increase the degree of fog in the image. The fog in gold-selenium or gold-tellurium sensitization is more remarkable than that in gold-sulfur sensitization. Accordingly, the fog caused by a selenium or tellurium sensitizer should be reduced particularly in the case that the selenium or tellurium sensitizer is used in combination with a gold sensitizer.
By the way, chalcogen compounds have also been used as silver halide solvents, silver halide ripening agents, antifogging agents or stabilizers. For example, U.S. Pat. Nos. 4,111,697 (Pollet et al), 4,782,013 (Herz et al) and Japanese Patent Provisional Publication No. 3(1991)-91735 disclose selenium or tellurium compounds. The disclosed compounds are effective as silver halide solvents, silver halide ripening agents, antifogging agents or stabilizers, but are not available as chalcogen sensitizers.